The present invention relates to the transmission of an alert message from a base station to a mobile station. More particularly, this invention relates to improving the transmission of an alert message from the base station to the mobile station.
In a typical wireless communication system, such as a cellular or Personal Communications Systems (PCS) network, a base station system having a controller and a plurality of transmitters and receivers communicates with a switching center and with a plurality of mobile communication units. Multiple access wireless communication between the base station and the mobile station occurs via RF channels which provide paths over which communication signals such as voice, data and video are transmitted. Base-to-mobile station communications are said to occur on a forward or down-link channel, while mobile-to-base station communications are referred to as being on a reverse or up-link channel.
To conserve the battery power of a mobile station used in a cellular or PCS network that employs Code Division Multiple Access (CDMA), the mobile station may operate in a xe2x80x9cslotted modexe2x80x9d when not handling telephone calls or when not receiving information about the network, such as information about the serving cell and neighboring cells. While operating in the slotted mode, the mobile station periodically monitors only those time slots on a forward-link paging channel (PCH), which are assigned to the mobile station to determine whether a base station is attempting to deliver an incoming call or message to it. The PCH slots are determined by using a hashing function that is based on mobile station-specific International Mobile Station Identity (IMSI). The IMSI is 34-bit binary encoded representation of the telephone number of the mobile station as per TIA/EIA IS-95, hereby incorporated by reference. At other times, the mobile station remains in an idle or standby state. However, because the PCH is shared by many mobile stations and carries both time slotted and non-time slotted messages, the PCH may carry information not intended for the mobile station in the mobile station""s time slot. A typical mobile station that employs a PCH is NOKIA(copyright) 6185 of Nokia Americas, 6000 Connection Drive, Irving, Tex. 75039, which monitors appropriate time slots on the PCH based on its IMSI. The mobile station must be periodically activated to monitor its assigned time slots, regardless of whether that time slot contains information intended for the mobile station or not. Thus, the mobile station needlessly consumes power when the base station sends a slotted message not intended for the instant mobile station because the mobile station must monitor every time slot assigned to it and may be required to monitor up to two time slots in every slot cycle.
To further conserve power, a new forward-link channel called quick paging channel (QPCH) has been added as part of the TIA/EIA IS-2000 wireless communication standard, hereby incorporated by reference. This new standard, which defines the over the air-interface for third generation CDMA system, corresponds to protocol revision 6. The QPCH is employed by the base station to alert an idle mobile station to activate and receive information carried by the PCH starting from its next assigned PCH time slot. Starting from the second phase of IS-2000, which corresponds to protocol revision 7, QPCH could also be used to indicate messages arriving in Forward Common Control Channel (F-CCCH). The QPCH is also divided into time slots comprised of pairs of bits which are known as paging indicator (PI) bits as well as into reserved and/or configuration change indicator (CCI) bits. For a given mobile station, the positions of the two PI bits within the QPCH slot are determined by a hashing algorithm employed by the base station and the mobile station. The mobile station turns on at the time that it is to receive its assigned PI bits on the QPCH. When both PI bits have been set to the value ON by the base station, the mobile station remains active to receive slotted information transmitted over the PCH. Alternatively when one or both of the PI bits have been set to the value OFF, the mobile station returns to its idle state until the next time that it is to receive PI bits.
However, the QPCH PI bits assigned to a specific mobile station may precede the corresponding PCH time slot by as much as 100 ms so that it is possible for the base station to turn ON the two PI bits assigned to a respective mobile station to indicate the presence of a slotted message in the paging channel and then be unable to send the message in the corresponding PCH time slot because of PCH overload. When the base station turns both PI bits ON to alert a single mobile station that a message is forthcoming, there is always a possibility that other mobile stations may be falsely alerted because all possible phone numbers are assigned to only a limited number of PI pairs. Thus, the battery power of a mobile station may be needlessly consumed when the mobile station incorrectly receives one or both PI bits on a PCH time slot when the PI bits should be received on a PCH time slot on another mobile station.
Accordingly, there exists a need for an improved way of computing the location of PI bit to enable the base station to alert the correct mobile station.
The present invention provides an improved method of alerting a remote device in an idle state over a channel. A value R1, which is the location of an initial PI bit within an initial half of at least one time slot on the channel, is computed based on a hashing algorithm defined in IS-2000 that uses information about the remote device. The initial indicator bit is assigned a binary value by the base station. Then, a value R2, which is the location of a further bit on a further half of the at least one time slot on the channel, is computed based on an improved hashing algorithm. Then, the further indicator bit is assigned a binary value by the base station. Then, the remote device in the idle state is alerted over the channel based on the assigned bit location R1 and further bit location R2.
Other aspects and features of the present invention will become apparent to those of ordinary skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.